Traditional aircraft taxi systems utilize the primary thrust engines (running at idle) and the braking system of the aircraft to regulate the speed of the aircraft during taxi. Such use of the primary thrust engines, however, is inefficient and wastes fuel. For this reason, electric taxi systems (i.e., traction drive systems that employ electric motors) have been developed for use with aircraft. Electric taxi systems are more efficient than traditional engine-based taxi systems because they can be powered by an auxiliary power unit (APU) of the aircraft rather than the primary thrust engines.
In its simplest form, a crew member may manually steer the aircraft during an electric taxi maneuver using a flight deck controller (e.g. a tiller) while looking out a window. In this case, the crew member utilizes their best judgment regarding execution of their taxi maneuver. An improvement over this process is provided by a visual guidance system wherein a crew member enters airport parameters such as airport congestion, the visual guidance system determines the best taxi path, subject to ATC clearance, and presents it on a cockpit display along with instructions as to the best way to navigate the aircraft along the suggested taxi path; e.g. speed, steering, when to turn thrust engines off and turn electric drive motors on, etc. ATC clearance can include taxi route, assigned take-off or landing runway, hold points etc. and is considered in the calculated path.
While effective, the above described visual guidance system exhibits certain inefficiencies. For example, variations in complying with display guidance instructions, even in the neighborhood of a few seconds, may decrease fuel savings; e.g. a pilot waits a short time before turning thrust engines off. The pilot may execute faster turns than necessary resulting in increased tire wear, or brake more often than necessary causing unnecessary wear and tear on the braking system. In addition, some actions that would increase efficiency are too subtle for the crew to recognize and manage; e.g. optimum acceleration of the aircraft during taxi.
In co-pending U.S. patent application Ser. No. 13/463,657 entitled “Electric Taxi Auto-Guidance and Control System” and assigned to the assignee of the present invention, an auto-guidance and control system for use in conjunction with an aircraft taxi system is described which may be operated in an automatic mode (auto-mode) or a manual mode should the automatic mode become inoperative or otherwise unavailable. The aircraft obtains aircraft status data and accesses airport feature data. This data is provided to a processor which generates taxi guidance information that is displayed. The processor generates taxi path commands that are provided to taxi path controllers in the auto-mode. The display and operation in the auto-mode must be monitored by the crew including viewing out the windows, to rapidly detect any failure of the auto-mode guidance system to avoid potential mishaps.
Certification authorities such as the FAA have great concern for passenger and crew safety. Thus, they provide regulations and design guidance so as to achieve a high degree of availability and integrity to ensure safety. That is there must be assurances that equipment must work when needed (i.e. be available) and not suffer undetected failures nor generate misleading guidance that cannot be remedied by pilot intervention (i.e. operate with high integrity).
For example, if an event such as loosing automatic or visual guidance in bad weather is identified by certification authorities and aircraft operators as “major” because it could significantly increase crew work load, the authorities would require that such an event have a probability of occurrence of no more than 10−5 per flight hour. A more serious event may be declared “hazardous” and have a probability of occurrence of no more than 10−7 per flight hour. These requirements are set forth in FAA specification AC25-1309.
In view of the foregoing, is would be desirable to provide an electric taxi automatic guidance and control system capable of guiding and controlling an aircraft during a taxi maneuver in severe weather conditions and limited visibility with virtually no crew involvement.